Buried pipelines that transport fluid products such as natural gas, crude oil, and other petroleum products are subject to damage due to physical forces applied to them, and due to chemical and electrolytic action. To assure that a pipeline is safe for continued operation it is periodically inspected for flaws by nondestructive testing apparatus which is carried through the interior of the pipeline by means of a pig. The pig is propelled through the pipeline by the fluid being transported therethrough.
One of the more serious flaws that potentially may make the pipeline unsafe for continued operation is a longitudinally extending anomaly in the wall of the pipe. Apparatus for detecting longitudinally extending flaws in a buried pipeline are disclosed in U.S. Pat. Nos. 3,238,448 by Wood et al., and 3,483,466 by Crouch et al.
Although the devices disclosed in those patents are useful, the device shown in U.S. Pat. No. 3,238,448 requires that a power source carried on the apparatus rotate a portion of the apparatus that carried the flaw detection means. Also, slip rings are required to couple detected flaw signals and electrical power to and from other portions of the apparatus. These requirements add complexity to the design and maintenance of the apparatus. Satisfactory operation of slip rings in a pipeline environment is particularly troublesome. In order to minimize the complexities mentioned above, the device of U.S. Pat. No. 3,483,466 was constructed in such a manner that longitudinally extending anomalies could be detected without the need for mechanically rotating a portion of the apparatus. This simplified the device and eliminated the need for the additional power source and slip rings associated with the rotating portion. However, because optimum sensitivity of detection of longitudinally extending anomalies is achieved when the flux leakage detector passes transversely across the longitudinally extending anomaly, the device of U.S. Pat. No. 3,483,466 does not provide optimum sensitivity of detection.
Additionally, in the prior devices discussed above, the search shoes which house the flaw detecting elements, such as search coils, are supported by mechanisms which involve moving and sliding parts and springs. The mechanisms are subject to corrosion and considerable wear, damage, and even loss when they encounter mashes, weld "icicles", valves, and traps, for example.